In the art of making paper, a cellulose fibre pulp slurry first is prepared by a known method. In order to produce high quality paper from the thus prepared pulp slurry, the brown stock must be bleached. The difficulty in the bleaching process is to remove the coloring matter without at the same time attacking the cellulose. In order to achieve this operation in the prior art, the bleaching is carried on in a number of stages. In the first stage, chlorine is added to the brown stock. Next, a sodium hydroxide alkaline wash is applied to remove the alkali-soluble chlorinated lignin compounds produced in the step of adding the chlorine. The next bleaching agent which is added is sodium hypochlorite. Following the sodium hypochlorite addition, chlorine dioxide is added to the stock which is again subjected to a sodium hyroxide alkaline wash to remove the alkali-soluble chlorinated lignin compounds produced in the preceding steps. Finally, in the last stage of the process additional chlorine dioxide is added to the stock to produce bleached stock from which high-quality paper can be made.
It will readily be appreciated that control of the bleaching process is desirable if high-quality results are to be produced. Some attempts have been made in the prior art to provide such controls. For example, chemical analysis of the stock at some point in the process affords an indication of the amount of bleaching compound which should be added to produce desired results. Such a control arrangement, however, is a secondary control rather than a primary control and suffers from all of the disadvantages of such a secondary control.
Chappelle U.S. Pat. No. 3,729,375, shows a system for controlling the addition of chlorine dioxide in the last stage of the bleaching process. Chappelle measures the brightness of the stock being fed into the last stage and compares the signal with a reference signal representing the desired brightness of stock leaving the last stage to obtain a difference signal. The difference signal is multiplied by a factor K to obtain a control signal which actuates a motor operated valve to regulate the addition of chlorine dioxide in the last stage. The factor K accounts for the temperature and rate of flow of the slurry and the chemical strength of the bleaching agent. Chappelle then measures the brightness of the stock leaving the last stage and compares it with the same standard to obtain a second difference signal which is used to modify the factor K to to reduce the second difference signal to zero. As is recognized by the patentee, the reaction time in this last stage is approximately 2 hours. In making his brightness measurements Chappelle uses a light source having a wavelength of 470 nanometers which is representative of the blue portion of the spectrum.
While the arrangement shown in the Chappelle patent discussed above affords some measure of control of the addition of the bleaching agent in a bleaching process, it is neither as stable nor as effective as is desired. The system is not particularly sensitive to small changes in brightness. It is relatively complicated and expensive for the result achieved thereby. It is susceptible to errors resulting from spurious factors such for example as consistency changes, species changes, black liquor contamination and to the addition of recycle bleached paper which is often added to the brown stock.
I have invented a continuous on-machine control of bleaching chemicals which overcomes the defects of systems of the prior art for controlling the addition of such chemicals. My system is not adversely effected by spurious factors such as consistency changes, species changes, black liquor contamination or the addition of recycled bleached paper to the brown stock. My system is more sensitive than are systems of the prior art. It affords primary control of the bleaching process. The sensing unit of my system has a very low temperature co-efficient. The output of the sensing unit of my system is reasonably linear with chemical concentration in the region of interest.